Accessibility: Add server rule to recommend image descriptions

[NMertsch]

From Discord: 25-accessibility.

Let's add the following to the server rules:

When posting pictures, please keep visually impaired attendees in mind. A short description can make a big difference.

[NMertsch]

Following this message, let's also encourage using alt text.

The UX and discoverability of this Discord feature is not great, so further instructions might be required.

Discord docs on the topic: https://support.discord.com/hc/en-us/articles/211866427-How-do-I-upload-images-and-GIFs#h_01GWWTHYJEV2S1WCDGFEMY21AQ